A circulator is often used with other optical devices to achieve certain optical functions. For example, a circulator can be used with a Brag Grating to extract an optical signal with a particular wavelength from a WDM optical signal. When an optical device such as a Brag Grating is manufactured using a Planar Lightwave Circuit (“PLC ”) or optical waveguide technologies, the optical device can become polarization sensitive. One method of using a PLC as a polarization insensitive device is to combine the PLC with a Polarization Maintenance (“PM ”) circulator. FIGS. 1a and 1b illustrate a four-port PM circulator 100 coupled to a PLC 60 through two PM fibers 20 and 30.
As shown in FIG. 1a, a randomly polarized signal S1, received from optical fiber 10, enters port one of PM circulator 100, and is separated into two light signals S2 and S3. Signal S2 exits from port 2 of PM circulator 100 with the y-polarization and enters PM fiber 20. Signal S3 exits from port 3 of PM circulator 100 with the y-polarization and enters PM fiber 30. Signals S2 and S3 are transmitted into PLC 60 through PM fibers 20 and 30 respectively.
As shown in FIG. 1b, optical signal S2′ with the y-polarization, received from PLC 60 through PM fiber 20, enters port two of PM circulator 100, and exits from port four of PM circulator 100 as a first component of signal S4 with the y-polarization. Optical signal S3′ with the y-polarization, received from PLC 60 through PM fiber 30, enters port 3 of PM circulator 100, and exits from port four of PM circulator 100 as a second component of signal S4 with the x-polarization. Combining the first and second components produce signal S4 with a random polarization.